A paired line is a conventional means of carrying telecommunications transmissions. A paired line is typically two balanced conductors individually insulated and twisted together along their length. A plurality of paired lines are usually bundled together to form a paired cable containing up to one hundred or more paired lines surrounded by a shield, wherein each paired line of the paired cable is capable of independently carrying a signal. The paired lines of a telecommunications cable are typically spiraled together in groups, having a characteristic spiral pattern termed the lay. A paired line is generally an effective telecommunications carrier, however, a fault can occasionally occur in a paired line limiting the effectiveness thereof. For example, a resistance fault can occur in a paired line when water invades the outer protective covering of a paired cable. The resulting resistance fault is a resistive contact between the two conductors of a paired line within the paired cable. The resistance fault is also often accompanied by a resistance from one of the two conductors of the paired line to ground or to another conductor of the paired cable. Another type of fault can occur when there is a discontinuity or break in the shield of the paired cable. The fault allows an increased coupling of extraneous voltages into one or more enclosed paired lines of the paired cable from nearby sources, such as power transmission cables. An open conductor fault or an underdesired bridged tap is a fault which interrupts current flow through a paired line or unbalances the paired line, diminishing the effectiveness of the paired line for telecommunications applications.
Such faults typically cause noise in the affected paired line that is extremely disruptive to the clarity of the telecommunications signal sent over the paired line. A disruptive to the clarity of the telecommunications signal sent over the paired line. A fault in a paired line can also cause other problems such as circuit failure, arc-over and corrosion. Therefore, it is desirable to locate and repair faults in a paired line, particularly a paired line in a telecommunications cable
Since telecommunications cables are not always readily accessible, often being buried below ground, noninvasive methods are preferred for locating faults in a paired line of a telecommunications cable. Tone location methods are conventional noninvasive means for locating a short fault in a paired line of a telecommunications cable using an audible tone as a locator signal. However, tone location methods are often ineffective for locating most resistance faults in a paired line. One reason for such ineffectiveness is a phenomenon termed carry-by. Carry-by occurs at faults that exceed a threshold resistance typically above about 2000 ohms. Most resistance faults have a resistance in a range between about 5000 to about 50,000 ohms. When a resistance exceeding the threshold resistance for carry-by is encountered at the resistance fault in a paired line, the current of the audible tone produces a voltage across the conductors of the paired line past the fault. This voltage causes a corresponding tone current to flow into pair capacitance beyond the fault. Thus, a tone will be audible beyond the resistance fault defeating the tone location method. The problem of carry-by is particularly acute when a high-frequency tone is present in the paired line because the magnitude of tone current flow past the fault is directly related to the frequency of the tone.
It is further noted that less tone current is typically driven through the paired line as the level of fault resistance increases. At relatively high levels of resistance, such as encountered in resistance faults, the level of tone current available for detection can drop below the background noise level present on a paired cable. Consequently, the background noise masks the location of the fault.
Alternate tone location methods have been developed in the telephone industry for detecting resistance faults, which overcome the above-described limitations caused by carry-by. For example, one alternate tone location method uses a 600-volt, breakdown test set to weld the conductors of the paired line together at the fault, which reduces fault resistance to zero in the paired line, thereby allowing effective tone location of the resistance fault. This tone location method, however, can cause extensive cable and equipment damage, particularly to plastic insulated cable and electronic central offices due to the high breakdown voltages used. Accordingly, this tone location method for overcoming fault resistance is disfavored.
Another alternate tone location method for locating resistance faults is disclosed in U.S. Pat. No. 4,291,204, wherein a tone locating device temporarily reduces the fault resistance to a low value by establishing a transient arc at the fault. Passing the fault locating tone through the low-resistance arc at the fault reduces tone carry-by, but does not entirely eliminate it. In addition, the conductors and equipment are susceptible to damage caused by formation of the transient arc, albeit to a lesser degree than the damage potential of the previously described tone location method.
Accordingly, it is an object of the present invention to provide a device and method for effectively locating faults in a paired line using a locator signal. In particular, it is an object of the present invention to provide a device and method for effectively locating resistance faults in a paired line using a locator signal. It is another object of the present invention to provide a device for effectively locating open faults or bridged taps in a paired line using a locator signal. It is yet another object of the present invention to provide a device and method for effectively locating shield break faults in a paired cable. It is another object of the present invention to provide a device and method for effectively locating faults in a paired line using a locator signal, wherein background noise is overcome. It is still another object of the present invention to provide a device and method for effectively locating faults using a locator signal, wherein the impact of carry-by is minimized. It is a further object of the present invention to provide a device and method for effectively locating faults in a paired line using a locator signal, wherein the device is fully portable in the field for expeditiously tracking relatively long lengths of the paired line. These objects and others are accomplished in accordance with the invention described hereafter.